This invention relates to lasers and more particularly to an apparatus and method for cavity dumping Q-switched radiation from a molecular laser to obtain pulses having submicrosecond duration with a high pulse repetition frequency.
The generation of pulses from a carbon dioxide laser having durations in the range between a few nanoseconds to three hundred fifty nanoseconds, suitable for laser radar or communication type applications, is highly desirable. Pulses having this duration are difficult to achieve in part from the recognized difficulty of modulating carbon dioxide lasers in general and the fact that this range of pulse duration lies between two common techniques for modulating carbon dioxide lasers, mode locking and intracavity Q-switching. The pulse duration of interest lies in the intermediate range where neither mode locking nor intracavity Q-switching will generate pulses which are of the desired duration. Mode locking techniques produce pulses of a few nanoseconds duration which are too short for many radar scenarios and have only a fixed pulse repetition frequency which may be unsuitable for many communication applications. Intracavity Q-switching techniques, either by a saturable absorber or by electro-optical modulators, have difficulty producing pulses of less than three hundred fifty nanoseconds at the full width half maximum point due to the dynamics of the upper energy level in the gain medium. A copending application entitled "Method for Cavity Dumping a Q-Switched Laser" Ser. No. 872,274 filed on even date herewith and held with the present application by a common assignee, discloses a method for the generation of short infrared pulses with adjustable pulse widths in the ten to three hundred nanosecond range utilizing an intracavity electro-optic modulator.
For applications requiring pulses of laser radiation having moderate power and high pulse repetition frequencies, passively Q-switched saturable absorbers have been employed with carbon dioxide lasers. Skolnick et al in U.S. Pat. No. 3,764,937 filed Apr. 26, 1972 and held with the present application by a common assignee discloses a SF.sub.6 saturable absorber to passively Q-switch a carbon dioxide laser having intracavity dispersive elements to prevent laser oscillations at lines for which the SF.sub.6 has a low or zero loss and to select the operating wavelength of the laser. In addition the cavity length is controlled to stabilize the output to a given pulse repetition frequency. For this configuration the intrapulse period is a function of the recovery time of the inversion of the carbon dioxide gain medium while the pulse tail is a function of the recovery time of the saturable absorber. A long pulse tail is undesirable for pulsed laser radar applications.
The generation of high peak power, temporally short (tens of nanoseconds) pulses from a carbon dioxide laser cannot be easily achieved by passive Q-switching. Additionally, with passive Q-switching, the pulse repetition frequency is determined by the resonator, gain medium and the saturable absorber medium dynamics. Active modulation offers the alternative of obtaining electronically controlled pulse widths and pulse repetition frequencies. Active intracavity modulation and Q-switching of a carbon dioxide laser can be accomplished in very short times at high rates with gaseous and solid crystalline modulators. However, fast switching to remove a high loss from the laser is not by itself sufficient to efficiently generate short, high peak power pulses. Rather a technique to cavity dump quickly the high circulating flux within a Q-switched laser is required.